[css-images] Add 'image-rendering: high-quality' value, per WG resolution.

Author: tabatkins

css-images/Overview.bs

@@ -1861,6 +1861,30 @@ Determing How To Scale an Image: the 'image-rendering' property {#the-image-rend
 			such as bilinear interpolation.
 			This is intended for images such as photos.
 
+		<dt><dfn>high-quality</dfn>
+		<dd>
+			Identical to ''image-rendering/auto'',
+			but with a preference for higher-quality scaling.
+			If system resources are constrained,
+			images with ''high-quality'' should be prioritized over those with ''auto'',
+			when considering which images to degrade the quality of
+			and to what degree.
+
+			<div class=note>
+				This value <em>does not</em> prevent the image quality from being degraded
+				when the system resources are constrained.
+				It merely expresses a preference that these images should receive extra scaling resources
+				relative to the ''image-rendering/auto'' images.
+				If all images on the page have ''high-quality'' applied,
+				it's equivalent to all of them having ''image-rendering/auto'' applied--
+				they're all treated the same.
+
+				To get the most value out of ''high-quality'',
+				only apply it to the most important images on the page.
+			</div>
+
+			Issue: The name of this value is currently being discussed.
+
 		<dt><dfn>crisp-edges</dfn>
 		<dd>
 			The image must be scaled with an algorithm that preserves contrast and edges in the image,
@@ -1873,9 +1897,9 @@ Determing How To Scale an Image: the 'image-rendering' property {#the-image-rend
 			to preserve a "pixelated" look as the image changes in size.
 	</dl>
 
-	Issue: Should we move the current ''auto'' behavior to a well-named keyword (''smooth''?)
-	and just define ''auto'' to be "whatever the UA wants",
-	which is what ''auto'' usually means?
+	Issue: Should we move the current ''image-rendering/auto'' behavior to a well-named keyword (''smooth''?)
+	and just define ''image-rendering/auto'' to be "whatever the UA wants",
+	which is what ''image-rendering/auto'' usually means?
 
 	This property does not dictate any particular scaling algorithm to be used.
 	For example, with ''image-rendering: auto'',